Chronic myelogenous leukemia is a rare myeloproliferative disorder characterized by the excessive development of white blood cells in the spongy tissue found inside large bones of the body (bone marrow), spleen, liver and blood. As the disease progresses, the leukemic cells invade other areas of the body including the intestinal tract, kidneys, lungs, gonads and lymph nodes.

There are two phases to chronic myelogenous leukemia. The first phase, or chronic phase, is characterized by a slow, progressive overproduction of white blood cells. An advanced phase is called the acute phase or blast crisis. At this point, over 50 percent of the cells in the bone marrow are immature malignant cells (blast cells or promelocytes). In the acute phase, the leukemia is very aggressive and does not respond well to therapy. Approximately 85 percent of all individuals with chronic myelogenous leukemia enter the acute phase.